A solid polymer type fuel cell is known as one form of a fuel cell. The solid polymer type fuel cell works at a lower temperature (approximately −30° C. to 120° C.), can be manufactured at a lower cost, and can be more compactly formed than fuel cells of other forms, and accordingly is expected as a power source of an automobile or the like.
As is illustrated in FIG. 3, the solid polymer type fuel cell 1 includes a membrane-electrode assembly (MEA) 2 as a main component, and makes the membrane-electrode assembly sandwiched between an anode-side separator 20 provided with a fuel (hydrogen) gas channel 21 and a cathode-side separator 30 provided with an air (oxygen) channel 31 to form one fuel cell 1 which is referred to as a single cell. The membrane-electrode assembly 2 has a structure in which an anode-side electrode 15a formed of a catalytic layer 13a and a gas diffusion layer 14a in an anode side is stacked on one side of a solid polymer electrolyte membrane 10 which is an ion exchange membrane, and a cathode-side electrode 58b formed of a catalytic layer 13b and a gas diffusion layer 14b in a cathode side is stacked on the other side thereof.
The catalytic layer 13 is formed of an electrode powder containing a catalyst-carrying conductor which is a carbon powder having a platinum-based metal catalyst carried thereon, for instance, and of an electrolytic resin, and is formed by applying the above described electrode powder which has been formed into an ink state or a pasty state together with the solvent on an appropriate substrate, drying the film and transferring the dried film to the electrolyte membrane 10 with a hot press or the like. A carbon paper or a carbon cloth is mainly used for the gas diffusion layer 14.
The output performance of the fuel cell (membrane-electrode assembly) is dependent on various physical properties of the components, and it is known that among them, hydrophilicity (wettability to water) of the catalytic layer is an important factor. When the hydrophilicity is excessively low, the conductive path of a proton cannot be formed and the output performance decreases in a less humidified period. When the hydrophilicity is excessively high, a flatting phenomenon tends to easily occur and the output performance decreases in a more humidified period. For this reason, studies are conducted for the purpose of imparting appropriate hydrophilicity to the catalytic layer. Patent document 1 describes a solid polymer type fuel cell which makes the catalytic layer contain a water-retaining agent including such sulfuric acid or phosphoric acid as to promote proton migration in the catalytic layer, and enables the catalytic layer at least in the oxygen pole side to retain 1 to 10 (mg/cm2) of a water content. In addition, Patent document 2 describes a solid polymer type fuel cell which is provided with an electrode catalytic layer containing a carbon black particle having a catalytic metal carried thereon and a polymer ion-exchange component (electrolytic resin), and which imparts such hydrophilicity that an amount of adsorbed water under a saturated water-vapor pressure at 60° C. is 150 cc/g or more to the above described carbon black particle constituting the catalytic layer.
[Patent document 1] JP Patent Publication (Kokai) No. 10-334922 A (1998)
[Patent document 2] JP Patent Publication (Kokai) No. 2002-100367 A (2002)